In the 1980's, it became apparent that many hazardous waste sites had received organic liquids, such as petroleum hydrocarbons, chlorinated solvents, creosote solutions and coal tars which had subsequently migrated into the subsurface beneath these sites. Once in the subsurface, these liquids dissolved and caused the contamination of ground-water supplies and then proved resistant to their quantitative removal by the remedial approaches available. These liquids are known to environmental scientists and engineers as non-aqueous phase liquids, or "NAPLs". NAPLs such as petroleum hydrocarbons, which are lighter than water, are identified as "LNAPLs", while those denser than water such as chlorinated solvents are known as "DNAPLs".
NAPLs are generally of sufficiently low aqueous solubility and volatility that their limited dissolution into ground waters or volatilization into gases has resulted in predictions of their residence in the subsurface for tens, hundreds or perhaps thousands of years. However, their toxicity is often such that their solubilities are many times the permitted maximum contaminant levels allowed by the U.S. Environmental Protection Agency in drinking water. For example, the most common NAPL contaminant found in ground waters beneath hazardous waste sites, the metal degreasing solvent trichloroethene, has an aqueous solubility of 1385 milligrams/liter but a maximum contaminant level of 5 micrograms/liter.
Partly because of their ubiquitous use in industry and commerce, low maximum contaminant levels and mobility in the subsurface in dissolved and gaseous states, NAPLs and NAPL constituents, such as benzene derived from gasoline, have come to occupy a central place in the technical and regulatory processes associated with the characterization and remediation of hazardous waste sites. In addition, NAPLs have become the focus of this concern because of the extreme difficulty in detecting their presence. In the context of this discussion, "detection" means the act of inferring the amount, location and/or composition of the NAPL. In recent years, a number of knowledgeable observers have commented on the cost and impracticality of detecting DNAPLs using conventional site-characterization techniques. See, for example, Huling and Weaver, DNAPL site evaluation, Project Summary, EPA/600/SR-93/022, U.S. Environmental Protection Agency, R.S. Kerr Environmental Research Laboratory, Ada, Okla., 74820 (1993); Cohen and Mercer, Dense nonaqueous phase liquids, Ground Water Issue, EPA/540/4-91-002, U.S. Environmental Protection Agency, R.S. Kerr Environmental Research Laboratory, Ada, Okla., 74820 (1989); MacKay and Cherry, "Groundwater Contamination:Pump-and-treat Remediation," Environmental Science and Technology, 23(6):630-636 (1993).
However, despite the expenditure of billions of dollars annually by the U.S. Government through the Environmental Protection Agency (for instance, in the implementation of Superfund), the U.S. Department of Energy (implementing the Environmental Restoration Program), the U.S. Department of Defense (implementing the Installation Restoration Program), as well as private corporations, the U.S. Environmental Protection Agency reported in April, 1993 (Cohen and Mercer, 1993) that "relatively little effort has been expended on developing new site-characterization tools or methods for DNAPL sites." This situation has resulted in substantive problems for DNAPL site characterization, in particular because of the tendency of DNAPLs, due to their density and viscosity, to migrate both vertically and laterally from their point of entry into the subsurface to considerable depth. Consequently, DNAPLs are "largely undetected and yet are likely to be a significant limiting factor in site remediation" (Huling and Weaver, 1991).
It follows from the sparing solubility and volatility of NAPLs that, generally, the vast majority of the mass of an organic liquid released to the subsurface may remain in the NAPL form. Relatively minuscule concentrations will be present in the dissolved and vapor states, but it is these less important phases that are generally monitored for compliance with regulations concerning the performance of the remedial operations at a site (see, Environmental Protection Agency, "General methods for remedial operations performance evaluations," EPA/600/R092/002, R.S. Kerr Environmental Research Laboratory, Ada, Okla. 74820 (1992)).